1. Field of the Invention
The present invention relates in general to lock-up torque converters and more particularly to improvements in clutch-slip control arrangements thereof.
2. Description of the Prior Art
A lock-up torque converter has two operation modes, i.e., an operation mode (converter state) in which an input element (usually a pump impeller) is driven by an engine to circulate working fluid which in turn drives under the reaction of a stator (reaction element) an output element (usually turbine runner) with an increased torque and another operation mode (lock-up state) in which a lock-up clutch is engaged to drivingly interconnect the input and output elements and thereby directly transmit a torque from the input element to the output element, and is adapted to take the former operation mode at a relatively low engine rpm range where the engine encounters an engine rpm variation problem and a requirement of torque increase and the latter operation mode at the remainder of the engine rpm range. Accordingly, the lock-up torque converter can reduce, as compared with the common torque converter having the former operation mode only, the engine fuel consumption by the amount corresponding to an reduction in slip between the input and output elements at a high engine rpm range (high vehicle speed range).
However, in the lock-up torque converter of the kind adapted to take the above two operation modes only, a lock-up vehicle speed at which engagement or disengagement of the lock-up clutch is performed must be set to a considerably high value so that upon engagement or disengagement of the lock-up clutch the resulting engine torque variation becomes small enough not to cause any vibrations of the vehicle body, resulting in that a lock-up operation range in which the lock-up clutch is held engaged is too narrow to reduce the engine fuel consumption to a satisfactory extent.
In order to solve the above problem, as is disclosed in U.S. Pat. Nos. 3,966,031 and 4,002,228, a clutch slip control technique has been proposed, which still encounters the engine rpm variation problem to some extent, wherein at a certain low engine rpm range in which an engine output torque is sufficiently large, the lock-up clutch is brought into slipping engagement while being limited for slip so that the engine torque variation is so small as to be negligible.
The clutch-slip control arrangements disclosed in the above mentioned U.S. Patents are provided with a variable opening orifice of which the opening is determined depending upon a force (turbine torque or differential pressure on opposite sides of turbine runner) variable with variation of a slip between the input and output elements of the torque converter, whereby a lock-up release pressure is varied to change an engagement force of the lock-up clutch, which clutch is actuated by the differential pressure between the lock-up release pressure and the pressure within the torque converter (converter pressure), so that the above mentioned slip is adjusted to a suitable value.
However, the above mentioned kind of clutch slip control arrangement has a problem that the clutch slip control effected thereby is unstable for the reason as will be described hereinafter. Description of the reason being made with reference to the U.S. Pat. No. 4,002,228 in which the turbine torque is utilized as the aforementioned force for determining the opening of the variable opening orifice and the turbine torque T varies in such a relation to the clutch slip "s" as shown in FIG. 11, so that upon occurrence of slip variaton .DELTA.s the turbine torque is caused to vary by the amount of .DELTA.T. On the other hand, the variation characteristic of the opening S of the orifice in relation to the turbine torque T is represented by the line "a" in FIG. 12. The inclination of the line "a" is determined depending upon the spring constant of springs disposed between two relatively rotatable members on the turbine side and the output side in such a manner as to resist the relative movement thereof and is set so that a suitable slip amount (slip amount of 60 rpm when the rotation of the torque converter output shaft is 1000 rpm) is obtained. In FIG. 12, the line "b" represents the variation characteristic of the lock-up release pressure P.sub.L in relation to the opening of the variable opening orifice S.
Now description being made to the case in which a turbine torque variation as represented by .DELTA.T in FIG. 11 occurs, such an opening variation of the variable opening orifice as is represented by .DELTA.S.sub.1 in FIG. 12 occurs respondingly, so that the resulting lock-up release pressure variation becomes so large as is represented by .DELTA.P.sub.1 in the same figure. On the other hand, the feed back amount of the clutch-slip control arrangement varies as a function of .DELTA.P.sub.1 /.DELTA.T, so that when .DELTA.P.sub.1 is so large as above, the feed back amount becomes excessively large. In the meantime, the clutch-slip control arrangement delays in operation in such a manner as to cause variation of the lock-up release pressure .DELTA.P.sub.1 with a responsive delay T.sub.1 and an action delay T.sub.2 as shown in FIG. 14, so that in the case where the feed back amount is so large as mentioned above, the rotational speed of the torque converter input element as represented by the curve "c" in FIG. 13 varies largely relative to the rotational speed of the torque converter output element as represented by the curve "d", causing the slip amount represented by the difference in rotational speed between the input and output elements to hunt and thus making the slip control unstable.
In order to solve this problem, it is considered that the spring constant of the aforementioned springs is increased for thereby making the inclination of the opening degree variation characteristic "a" of the variable opening orifice steeper as represented by the line "a'" in FIG. 12. When this is the case, the opening degree variation .DELTA.S.sub.2 of the variable opening orifice in response to a given variation .DELTA.T of the turbine torque is made smaller and therefore a lock-up release pressure variation .DELTA.P.sub.2 for a given turbine torque variation .DELTA.T is made smaller, causing the feed back coefficient .DELTA.P.sub.2 /.DELTA.T to become smaller. It therefore becomes possible to prevent hunting of the rotational speed of the torque converter input element and make the clutch slip control stable. However, when this is the case, the slip amount becomes too large (e.g. 200 rpm) and is largely deviated from a suitable value, thus making it impossible to attain a desired clutch-slip control.